Recent years have seen increase in amount of data in data of music, pictures, information, or the like along with progress of digital technology for electric appliances. Demands for large-capacity memory devices are increasing to store data having a great amount of data.
Examples of a large-capacity memory device include a variable resistance nonvolatile memory device. The variable resistance nonvolatile memory device is a memory device which uses for a memory cell a variable resistance element with resistive status that reversibly changes between a high resistance state and a low resistance state according to an applied electric pulse and retains the resistance state.
A typical variable resistance element has a relatively simple element structure including a first electrode, a second electrode, and a variable resistance layer which is positioned between the first electrode and the second electrode and has resistive status that reversibly changes according to an electric pulse applied between the first electrode and the second electrode (see patent literature (PTL) 1 and PTL 2, for example).
Furthermore, the variable resistance element has been attracting attention not only because of its relatively simple element structure but also because it is easy to increase its density and to ensure consistency with a conventional semiconductor process.
Such variable resistance elements are divided into two general types according to variable resistance materials that are materials for variable resistance layers.
A variable resistance element of one type includes, as a variable resistance material, a material having a perovskite crystal structure (e.g., Pr1−xCaxMnO3 (PCMO), La1−xSrxMnO3 (LSMO), or GdBaCoxOy (GBCO)) disclosed in PTL 1 and so on.
Furthermore, a variable resistance element of the other type includes a binary metal oxide as a variable resistance material. Compared to the material having a perovskite crystal structure, composition and structure of binary metal oxides are so simple that composition control and film formation in manufacturing can be easily performed. Furthermore, binary transition metal oxides advantageously have rather favorable consistency with semiconductor manufacturing processes, and thus there has been considerable research on binary metal oxides.
According to recent studies on mechanism of resistance change in a binary metal oxide, some researchers believe that a likely cause of such resistance change is deposition and dissolution of conductive filaments in the metal oxide (see PTL 2 and Non Patent Literature (NPL) 1, for example).